There has been proposed time measuring circuitry in which outputs of inverters in a ring oscillator are held by a flip-flop train at a predetermined input timing and performs time measurement by using the held data.
However, for the inverters in the ring oscillator, the delay time is not constant due to device variation or the like, so that linearity of the time measuring circuitry becomes worse. In order to improve the linearity of the time measuring circuitry, it is required to perform PLL control, resulting in complicated circuitry. Even if the PLL control is performed, a certain degree of nonlinearity inevitably remains.